Parity nonconservation in polarized-neutron transmission throughLa139

Parity nonconservation has been studied in the transmission of longitudinally polarized epithermal neutrons incident upon a ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ target. Previous experiments have demonstrated the existence of a large value of the longitudinal asymmetry in the 0.734-eV p-wave resonance of the compound nucleus ${}_{}{}^{140}{}_{}{}^{}\mathrm{La}$, but report conflicting values for the measured asymmetry. We report the results of an experiment at the Los Alamos Neutron Scattering Center (LANSCE) which measured the asymmetry by two independent methods. One of these methods utilizes a conventional polarizer and a single lanthanum target. The other method uses an unpolarized beam with two lanthanum targets, and therefore does not require a separate measurement of the beam polarization. The asymmetry values determined from these two methods (0.1015±0.0045 and 0.0955±0.0035, respectively) are compared to previously measured values. Measurement of the asymmetry of the 0.734-eV resonance in ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ provides a highly accurate method of determining the polarization of an epithermal neutron beam.